In an electron microscope equipped with an x-ray detector, this detector is mounted close to the objective lens and also close to a specimen. As the detector is placed closer to the specimen, the solid angle is increased and thus the detection efficiency can be enhanced. Such an electron microscope is disclosed in, for example, U.S. Pat. No. 4,910,399. However, if this detector has a protective film on the front surface, and if the protective film is very thin, then it is necessary to insert the detector into a valve mounted inside a detecting unit except when the detector is in use. Normally, the position of the detector is made different, depending on whether it is in use or not. When the detector is not in use, the detector is placed remote from the objective lens.
More specifically, as shown in FIG. 3, a specimen 16 is held by a specimen holder 19 between the top polepiece 15a and the bottom polepiece 15b of the objective lens. An electron gun (not shown) emits an electron beam 17 toward the specimen 16. A specimen image is projected onto a fluorescent screen 18. An x-ray detector 33 is contained in a protective housing 32. The detector 33 is cooled by refrigerant supplied from a refrigerant tank 31. The detector 33 is accommodated in a valve 34. The detector 33 is placed at a first position remote from the specimen in order to protect the detector 33 when it is out of use. On the other hand, when the detector 33 is in use, it should be placed at a second position in proximity to the specimen 16 to increase the detection efficiency. Therefore, when x-rays from the specimen 16 should be detected by the detector 33, the valve 34 is opened. Then, the detector 33 is moved to the second position close to the specimen. In order to carry out the above movement, a roller 35 in contact with the protective housing 32 is provided. The roller 35 is connected to a motor 36. The motor 36 is controlled by a driving circuit 37. A switch 38 is connected to the driving circuit 37 for instructing the movement of the detector 33. When the position of the x-ray detector 33 is changed, the position on the specimen irradiated by the electron beam 17 is an error. Therefore, the brightness of the image formed on the fluorescent screen 18 decreases. Further, at the same time the image itself deviates or errors from a first position indicated by P to a second position indicated by Q in FIG. 4 for the following causes. First, the x-ray detector 33 is made of a nonmagnetic material, but a slight amount of residual magnetism exists. When the detector is cooled by the refrigerant, the surface is frozen. This frozen surface is electrically charged by the electron beam. Secondly, a magnetic substance is contained in the mechanism which moves the x-ray detector. The amounts of these errors are relatively small. Hence, where the size of the electron probe is large, as encountered in the prior art instrument, no problem occurs. However, where the size of the electron probe is small, the slight errors become serious, since it is difficult to locate the analyzed point at a desired point after the x-ray detector has been inserted.